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LOK PAO

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AKA: Priborno-agregatniy otsek. Status: Operational 1971. Thrust: 4.17 kN (937 lbf). Gross mass: 5,646 kg (12,447 lb). Unfuelled mass: 2,494 kg (5,498 lb). Specific impulse: 314 s. Height: 4.21 m (13.81 ft). Diameter: 2.15 m (7.05 ft).

Celestial navigation sensors and digital computer for navigation. Volna-20 fuel cells had a mass of 70 kg and could provide 1.5 kW of power at 27 V for 500 hours. The cells were fed by 600 kg of liquid hydrogen and oxygen. The water generated by the reaction of the hydrogen and oxygen was used by the crew.

The PAO consisted of several distinct sub-modules:

• PO (Perekhodnoy Otsek) - Transition compartment. This was a truncated cone connecting the SA to the aft of the spacecraft. It was depressurized and housed four pairs of larger translation engines canted slightly forward so that they went through the spacecraft's center of gravity. They allowed up/down/right/left maneuvers in three dimensions during docking operations. Propellant was provided from the Block I propellant system.
• PO (Priborniy Otsek) - Equipment compartment. This was a pressurized cylinder with forward and aft hemi-ellipsoidal pressure bulkheads, similar to that used in other Soyuz models. It contained avionics - communications, telemetry, and command-link units. Most important of these was the digital computer of Pilyugin's guidance system by developed by NIIAP. This guided not only the LOK, but also the entire L3 stack throughout all flight phases from translunar injection from earth orbit through to return to earth. The guidance system was provided position and orientation data from a full-time inertial navigation unit, also by Pilyugin.
• AO (Agregatniy Otsek) - Engine compartment. On the LOK, this was a cylinder fitting and carrying loads over the outside of the spherical propellant tank of the Block I trans-earth injection propulsion system. It was covered on the outside with 24 extremely fragile hinged thermoregulation panels.
• Block I - This integrated propellant tank/engine assembly was developed by Isayev. It contained 3,152 kg of N2O4/UDMH propellants contained in a spherical 1.9 m diameter tank with a common bulkhead between the oxidizer and fuel. This tank fed two primary engine assemblies. The two-nozzle S5.51 engine, had a total thrust of 3,388 kgf, a specific impulse of 314 seconds, and was used for the 1100 m/s trans-earth injection maneuver at the end of lunar orbit operations. The S5.51 was flanked by the two smaller nozzles of the S5.53 engine. With a total thrust of 417 kgf and a specific impulse of 296 seconds, this was used for lunar orbit maneuvers and mid-course corrections on the way back to earth. It was rated for 35 firings. The same engine was used in the lunar flyby spacecraft (Soyuz L1, L1E, L1P, and L1S) and was a close relative of the two-nozzle back-up engine used on the Soyuz 7K-OK orbiter. The Block I propellants were also fed to very 16 small thrusters for roll, pitch, and yaw mounted on the base of the EO. These were used for orientation on the flight back to earth.
• EO (Energo-Otsek) - Power Module. This compartment was the flared base of the LOK. It connected the LOK to the payload shroud surrounding the LK and Block D stages during the trip to the moon until the LK/Block D separated from the LOK. The center of the space was taken up with the shrouded engine assembly of the Block I. Arranged within the periphery of the truncated cone were four fuel cells, their associated liquid oxygen and hydrogen tanks, and a refrigerator unit to keep the cryogenic liquids in liquid state. The outside of the EO was covered by tubular radiators of the refrigeration unit.

Electric System: 750.00 kWh. Electric System: 1.50 average kW.